1) get cliend id. under console/API Manager/Credential/Credential [https://console.cloud.google.com/apis/credentials?project=java-helloworld-157322]
2) set web client id in Server side(Java/Python code).
3) set client-id in frond-end(app.js or other JavaScript code).
4) allow JavaScript origin in Console, under console/API Manager/Credential/Credential/Domain verification.